LEITIS. CROSBY
Photodecomposition of Trifluralin Eriks Leitisl and Donald G. Crosby*
Trifluralin decomposed readily in water or aqueous methanol a t sunlight wavelengths to form a multitude of products. In addition to the minor dealkylated intermediates, the principal product under acidic conditions was 2-amino-6-nitron,ci,cu-trifluoro-p-toluidine.At alkaline pH, 2ethyl-7-nitro-5-trifluoromethylbenzimidazole represented about 80% of the photolysis products within 24 hr. Under all conditions, the highly
polar 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-trifluoromethylbenzimidazoline and 2-ethyl-7-nitro5-trifluoromethylbenzimidazole 3-oxide were present in significant amounts but were degraded by heat or further irradiation. The photochemical formation of benzimidazolines, benzimidazoles, and benzimidazole N-oxides conforms to a general mechanism which should apply to many dinitroaniline herbicides.
Trifluralin (2.6-dinitro-N,IV-dipropyl-cu,~,~-trifluoro-pethyl ether was an anhydrous analytical grade (Mallinckrodt). All other chemicals were used in the commercial toluidine, I ) is a selective preemergence herbicide representative of a growing list of N-substituted 2,6-dinitroanilform. 2-Ethyl-7-nitro-1-prop~l-5-trifluorometh> lbenzimidazole ines important to weed control. In 1970, 241,000 lb of tri(ZZZ). A solution of sodium sulfide nonahydrate (25 g, 0.1 fluralin was used in California, while cotton and tomatoes mol) and sodium bicarbonate (8.4 g, 0.1 mol) in 50 ml of alone employed almost 100,000lb there in 1972 (California Department of Agriculture, 1970, 1972). water was added dropwise to a stirred solution of VI (8.8 The dinitroanilines are very unstable toward ultraviolet g, 0.03 mol) in 300 ml of methanol at room temperature during 10 min. The mixture was stirred for 3 hr and (uv) light. For example, ten products were detected by gas-liquid chromatography (glc) when I was irradiated in poured into 1 1. of water. After 15 min, the suspension was filtered, and the residue washed thoroughly with water, anhydrous methanol (Day, 1963), and 2,6-dinitro-N-n-propyl-m,iu.cu-trifluoro-p-toluidine (VI) and 2,6-dinitro-cu,cu,cu- taken up in benzene, and dried. The dried solution was filtered, concentrated to about 200 ml, and held at 5" trifluoro-p-toluidine (XII) were tentatively identified by overnight. matching retention times. Irradiation of VI in heptane The precipitate was removed by centrifugation, and the with a high-pressure mercury arc lamp (McMahon, 1966) in turn provided 2-nitro-6-nitroso-cu,cu,cu-trifluoro-p-tolui-supernatant was repeatedly concentrated. The subsequent crops of crystals (3.5 g, 39%) were combined and recrysdine. Exposure of I in hexane or methanol to laboratory uv tallized twice from benzene-hexane, mp 79-81". Further light for 24 hr (Harrison and Anderson, 1970) gave products tentatively identified as VI, 2-amino-6-nitro-N-pro- purification by preparative glc (1%SE-30 on Chromosorb pyl-cu,tu,ci-trifluoro-p-toluidine, and 2-amin0-6-nitro-~V,~V- W) gave 2-amino-6'-nitro-,V-propSl-ct,tu,cu-trifluoro-p-toluidipropyl-cu,n,cu-trifluoro-p-toluidine. The products from dine eluting at 185", mp 82-83" (82-83", Emmerson and the photodecomposition of I on glass or soil (Wright and Anderson, 1966): ir 3452, 3378 (NHz), 1537, 1333 (NOz) c m - l ; nmr 6 7.8 (s, Ph), 7.1 (s, Ph). 6.3 ( s , "21, 4.1 (s, Warren, 1965) or in aqueous acetone (Messersmith et al., 1971) remained unidentified. NHz), 3.2 (t, CHz), 1.5 (m. CHz), 1.0 ( t , CH3). The diamine (0.3 g, 0.001 mol) was boiled with 10 ml of The outdoor application and subsequent movement of I propionic acid and 20 ml of 4 N HC1 for 4 hr, cooled. dican result in exposure to sunlight, especially where the luted with 120 ml of water, and neutralized with dilute herbicide might occur in irrigation water, and preliminary ammonium hydroxide. 111 was extracted into methylene experiments under these conditions indeed revealed rapid chloride, dried, concentrated, and purified by preparative photodecomposition to a large array of substances (Leitis glc (2% SE-30 on Chromosorb W) where it eluted at 180and Crosby, 1972). The purpose of the present work was 183" as a nearly colorless, viscous liquid which slowly to elucidate the products and pathways of this photodedarkened on standing: ir 1536, 1337 (NOz) c m - l ; mass composition in sunlight and to positively identify the spectrum m/e 301 (parent and base); nmr 6 8.2 (s, Ph), major products formed by uv irradiation of I in both aque8.1 (s, Ph), 4.3 (t, CH2). 3.0 (quadruplet, CH2), 1.7 (m, ous and organic media. CHz), 1.5 ( t , CH3),and 0.9 ( t , CH3); uv A,,,,, 312, 226 nm. EXPERIMENTAL SECTION 2,6-Dinitro-N-n-prop?l-a,n,cu-trifluoro-p-toluidine ( V I ) , 4-Chloro-3,5-dinitro-n,n,cu-trifluorotoluene (13.5 g, 0.05 Synthesis and Purification of Standards. Technical mol) in a minimum amount of benzene was added droptrifluralin (95%) (Eli Lilly and Co., Indianapolis, Ind.) wise to a stirred solution of n-propylamine (12 g, 0.15 mol) was recrystallized from absolute ethanol until homogein 100 ml of benzene, stirred at 25" for 4 hr, the benzene neous on thin-layer chromatography (tlc) and glc, mp 4748" (48.5-49", Probst e t a1 , 1967). 4-Chloro-3,5-dinitro- washed with water. and the washed solution dried and concentrated until the product precipitated. Recrystallia,o,tu-trifluorotoluene (Eli Lilly) was recrystallized from zation from benzene provided 12.1 g (83%) of VI. mp ethanol. mp 55-57" (56-58", Hall and Choo-Seng, 1972). 62.5-63" (55-58". Hall and Choo-Seng, 1972): ir 3356 Technical dl-2-aminobutyric acid (Eastman Organic ( S H ) , 1548, 1326 (NOz) c m - l ; nmr 6 8.6 (s, KH), 8.4 (s, Chemicals) was recrystallized twice from aqueous ethanol, Ph), 3.0 (t, CHz), 1.7 (m, CHZ), and 1.0 (t, CH3); mass mp 298-301" (304", Patai and Zabicky, 1971). spectrum m,ie 293 (parent), 264 (base). Acetone, benzene, chloroform, ethyl acetate, hexane, 2 - E t h y l - 7 - n i t r o - 5 - t r i f l u o r o m e t ~ ~ l ~ e n ~ i ~ i d a3-Oxide zole methanol, and methylene chloride were redistilled. and (VU). To 4-chloro-3,5-dinitro-cu,cu,cu-trifluorotoluene (10 g, 0.037 mol) in 110 ml of ethanol was added a solution of a-aminobutyric acid (5.15 g, 0.05 mol) and sodium bicarDepartment of Environmental Toxicology, University of bonate (5.15 g) in 55 ml of water; the mixture was shaken California, Davis, California 95616. at 25" for 2 hr, allowed to stand overnight, concentrated to Present address: Pacific Biomedical Research Center, remove most of the ethanol, and diluted to 200 ml with University of Hawaii. Honolulu, Hawaii. 842
J. Agr. Food Chem.. Vol. 22, No. 5. 1974
PHOTODECOMPOSITION OF TRIFLURALIN
water. Unreacted reagent was filtered off and the filtrate extracted with methylene chloride and acidified. The precipitated product was chromatographed on a silicic acid column with chloroform-ethanol (18:l) and the central eluate concentrated to dryness to provide 2.0 g (16%) of N (2', 6'-dinitro-4'- trifluoromethylphenyl)-2-aminobutyric acid as a bright yellow solid, mp 230-231" dec; ir 3356 (NHz), 1724 (C:=O), 1502, 1304 (NO21 c m - l ; nmr 6 8.5 (s, Ph), 2.7 (t, CH), 1.7 (CHz), 0.8 (t, CH3); the N-proton was masked by methylene protons. The intermediate acid (0.1 gi in 1400 ml of 0.5% acetic acid was irradiated at 300-400 nm in a preparative photoreactor (Crosby and Tang, 1969) for 1 hr, acidified to pH 2.0, and extracted with methylene chloride. The extract was subjected i o tlc on silica gel G (solvent B), and the band a t R l 0.16 was extracted with acetone; the residue remaining after evaporation of the extracts sublimed a t 150-160" (0.01 Torr) to provide VI1 as the sublimate, mp 215-216" dec: ir 3436 (NH), 2632, 3003-2849 (CH), 1538, 1337 ( S O 2 ) c m - - l ; nmr 6 8.5 (s, P h ) , 8.3 (s, P h ) , 3.4 (s, NH or OH), 3.0 (quadruplet. C H Z ) , and 1.4 (t, CH3); mass spectrum m / e 275 (parent), 259 (base). 2-Ethyl-7-nitro-S-trifluoromethylbenzimidazole ( X I ) was prepared (86% yield) in the same way as I11 and recrystallized from benzene-hexane, mp 150.5-151": ir 3344 (NH), 1520, 1335 (r\;O2) c m - l ; nmr 6 8.4 (s, Ph). 3.1 (quadruplet CH2). 2.1 (s, KH), 1.5 (t, CH3); mass m / e 259 (parent and base). 2,6-Dinitro-m,CY, a-trifluoro-p-toluidine (XU). A suspenwas sion of 4-chloro-3,5-dinitro-a,~,~-trifluorotoluene stirred with a tenfold excess of concentrated ammonium hydroxide at 50" for 2 hr and then at 70" for 2 hr. The precipitate was filtered off, washed several times with water, and triturated with a small amount of ether to provide a pale yellow solid, mp 142.5-144" (143-144", Hall and Choo-Seng, 1972): ir 3460, 3356 (KHz), 1548, 1362 (NO21 cm-1; mass spectrum m / e 251 (parent and base). 2-Amino-6-nitro-a,a,a-trifluoro-p-toluidine (XZZZ). A solution of sodium sulfide (25 g, 0.1 mol) and sodium bicarbonate (8.8 g. 0.1 mol) in 50 ml of water was added dropwise at 25-30' to XI1 (7.8 g, 0.03 mol) in 500 ml of methanol during 15 min. The mixture was stirred a t 25" for 2 hr, filtered, and poured into 1500 ml of water with stirring. The resulting precipitate was filtered off and washed with water; the filtrate was extracted with methylene chloride. and the extract washed with 3 N HC1 and then water. dried, and evaporated to dryness. The residue and precipitate were combined and recrystallized twice from benzene-hexane, with final purification by preparative tlc on silica gel G (solvent A ) to give a red-brown solid. mp 132.5-133.8" (127-128", Emmerson and Anderson. 1966): ir 3497, 3425, 3378, 3344 ("2). 1546, 1335 ( N 0 2 ) c m - l ; 5.0 (s, "2). Anal. Calcd nmr 6 7.8 (s, P h ) , 7.2 (s, "21, for C1H6N302F3: C, 38.0; H, 2.73; N, 19.0. Found: C, 37.7; H. 2.80; N,18.8. Chromatography. Tlc was carried out on 20 x 20 cm glass plates coated with 0.5 mm of silica gel G (Stahl) containing 17~o f zinc orthosilicate phosphor. The spots and bands were detected by fluorescence quenching under 254-nm light and by visual observation. Bands from preparative tlc were scraped off and extracted three times with warm acetone or ethyl acetate, and the suspensions filtered through a medium porosity fritted glass filter. If tlc was the final purification step, the extracts were dried over anhydrous sodium sulfate, concentrated t o 4-6 ml. and centrifuged to remove any silica. The solvent systems used for tlc separations are listed in Table I. Glc separations were performed on an F and M Model gas chromatograph 720 temperature-programmed equipped with thermal conductivity detector, 60 cm x 0.3 cm i.d. stainless steel columns containing either 1% SE-30 silicone gum on 60-80 mesh, acid-washed, DMCS-treated Chromosorb G or 2% SE-30 on identically treated Chro-
Table I. Solvent S y s t e m s f o r Thin-Layer C h r o m a t o g r a p h y Acetone-hexane C hloroform-ethanol C hloroform-ethanol-concd ammonium hydroxide E ther-hexane E thanol-hexane-benzene Acetone-hexane Acetone-hexane-toluene Benzene-petroleum ether
1:2 (v/v) 9: 1 100: 5 :0 . 5
Solvent A Solvent B Solvent C
3: 1 1:3:2 1:5 2:2:1 3: 1
Solvent D Solvent E Solvent F Solvent G Solvent H
mosorb W, programmed at 7.5"/min from 100 to 280", injection port and detector temperatures 250 and 300". respectively, and He flow a t 40 ml/min. The eluting compounds were collected from the exit port in glass capillary tubes. Spectra. Spectra were obtained under the following conditions: uv, Beckman DK-2A, methanol solution; nmr (60 MHz), Perkin-Elmer Hitachi R-2.0. in acetone-d, chloroform-d, or dimethyl-d sulfoxide, T M S internal standard; 100 MHz nmr spectra, Varian HA-100; ir, KBr discs or thin films. Perkin-Elmer 337; mass spectra, Varian M66 (probe); Finnigan 3000 fitted with a 1.2 m x 3 mm i.d. column containing 3% OV-17 on 60-80 mesh Chromosorb W. He flow, 10 ml/min; ionization voltage, 70 eV; high-resolution mass spectra, CEC 110. Irradiation. A preparative photoreactor (Crosby and Tang, 1969) simulating sunlight conditions provided workable quantities of photolysis products. Solutions initially containing 10-200 mg/l. of I in 10% aqueous methanol were irradiated for periods of a few seconds to 2 days. Air was bubbled in to provide agitation and oxygen, and reactor temperatures were maintained at 30-40". In some instances, the irradiated solutions were 0.01 M in sodium carbonate (pH -11.0). Irradiations in sunlight were conducted outdoors with 1-50-ppm suspensions of I in deionized or tap water in 4-1. borosilicate glass erlenmeyer flasks, closed with perforated aluminum foil, a t Davis, Calif., from June to September 1970. Irradiation times varied from several hours to 1 month, and the temperature of the suspensions occasionally reached a maximum of 48". Certain of the isolated photolysis products (11. 111. VI, VII, and XI) also were suspended in deionized water (10-50 ppm) and exposed to sunlight for 4 days. Isolation and Identification of Photolysis Products. The sunlight-irradiated solutions of I in water were adjusted t o p H 7.0-7.2 with sodium bicarbonate and extracted four times with methylene chloride (neutral extract). The extracted solutions were adjusted to pH 2 with dilute sulfuric acid and again extracted with methylene chloride (acidic extract). The separate extracts were dried (sodium sulfate). concentrated on a rotary evaporator at 30--50", and fractionated by preparative tlc. The irradiated solutions of identified photolysis products were worked up in the same manner, while solutions from the photoreactor (pH 5.5) were concentrated on the rotary evaporator below 50" before neutralization and extraction. Neutral Extracts. Preparative tlc (solvent A ) resulted in detectable bands, labeled 1-7 in order of decreasing R I , which were scraped off and eluted with warm acetone and/or ethyl acetate (Table 11). Band 1. The extracted material ( R , 0.69) was purified by glc (1% SE-30i: ir 3003-2915 (CH,), 1550-1323 (NO*) c m - l ; nmr 6 8.0 (s. Phi. 3.0 (t, CHz), 1 . 7 (m, CHz). 0.9 ( t . CH3). Band 2. The extracted material was purified by tlc (solvent A. R l 0.63, then solvent F, R , 0.49). followed by glc (1% SE-30). Ir, nmr, and mass spectra were the same as VI. Band 3. Rechromatography (solvent G. R I 0.64. then solvent H ) provided a major (Rf 0.36) and a minor (RI J. A g r . Food Chem.. Vol. 22. No. 5. 1974
843
LEITIS, CROSBY
Table 11. Trifluralin Photodecomposition Products Tlc band
Rf"
Elution temp, "Cb
MP, "C
Re1 amt, % c
Identity
~
3 4
0.69 0.63 0.56 0.50
5
0.36
6 7 1A
0.10 0.0
1 2
(1
0.05
160 156 240-260 250-261 171d
173, 183, 192O 160, 168, 171, 185d e
48 62 132-136 217 142 132 190-193 Dec Liquid 215-216 dec
2 5 5
2 25 5 20 20
I VI Azoxy compd? Azoxy compd? XI1 XI11 XIV Unidentified I1 VI1
Solvent A. h 1%SE-30. c 10% aqueous MeOH, 24 hr. 2% SE-30. e Decomposition products.
Table 111. T h e r m a l Decomposition P r o d u c t s of C o m p o u n d s I1 and VI1 _____-
.-
RJ valuea Solvent A
Solvent D
0.54
0.42 0.30 0.65
0.35 0.63
0 . 4gd
0.37
0.60 0.20 0.52 0.42
Elution temp, "C
Mass spectrum, parent m / e
Compound I1 183b 192h 173h Compound VI1 16OC 185c 17OC
MP, "C
Identity
301 317 315
Liquid Liquid Liquid
I11 IV V
273 275 257 259
202-204 138-142 167-171 150-1 51
VI11 IX X XI
Silica gel G. 1% SE-30. 2% SE-30. Solvent E. 0.39) fraction. The investigation of the major fraction will be described elsewhere. The minor fraction was purified by glc (2% SE-30) to give a yellow solid, mp 142". Ir spectrum was the same as XII. Band 4. Rechromatography (solvent G. R I 0.62, then solvent H) provided a major ( R t 0.26) and a minor (Rf 0.07) fraction. The investigation of the major fraction will be described elsewhere; the minor fraction provided a yellow-brown solid in amounts too small to characterize. Band 5. The extracted material ( R I 0.36) was subjected to tlc (solvent D) providing a major ( R , 0.21) and a minor band (Rl 0.41). The major one was extracted into ethyl acetate, the solution centrifuged and taken to dryness, and the red-brown solid recrystallized from benzene-hexane, mp 132-134". The ir spectrum was the same as VIII. Reaction with glacial acetic acid and 4 N HC1 as described for the synthesis of I11 gave 2-methyl-7-nitro-5-trifluoromethylbenzimidazole whose mp (205.5-207") and ir spectrum were identical with those of an authentic specimen. The minor band was extracted into ethyl acetate, concentrated, chromatographed on a silicic acid column, and eluted with benzene-hexane (1:2) to give an orange-red solid (XIV). mp 190-193": ir 3472, 3356 (NHz), 3012-2865 (CH,), 1543, 1337 (NOz) c m - l ; nmr 6 7.4 (s, Ph), 6.4 (s, Ph), 3.0 (s, NH); mass spectrum m / e 261 (parent). 246 (base; P - CH3). Anal Calcd for C I O H ~ O N ~ O C, Z F46.0; ~: H , 3.83: N, 16.1. Found: C, 46.2; H , 3.95; N,16.0. Band 6. The extracted material (Rl 0.10) was rechromatographed (solvent A, then solvent E, R I 0.28). extracted into acetone, and taken to dryness to provide a brown amorphous solid. It degraded during tlc or column chromatography to a more polar material, decomposed above l a " , and could not be sublimed or distilled under vacuum: ir 3484 ( N H ) , 3030-2874 (CHn), and 1389 (NOz?) cm-I. Band '7. Extracted material was rechromatographed with solvent A ( R , 0.0) and then twice with solvent C ( R , 0.21). The acetone eluate was concentrated, centrifuged to 844
J. Agr. Food Chem., Vol. 22, No. 5, 1974
remove insoluble material, and evaporated under a stream of nitrogen, and the dark red residue (11) was dried in vacuo at 25" over phosphorus pentoxide and paraffin shavings and stored in a desiccator in the dark. Alternatively, the entire neutral extract was chromatographed on a silicic acid column and all major products except IX were serially eluted with acetone-hexane at ratios of l : l O , 1:4, and 1:l (v/v), respectively. The upper third of the column containing IX was extruded, extracted twice with warm acetone and once with warm methanol, and the extracts were purified by preparative tlc twice with solvent C. I1 was very difficult to purify because it decomposed on standing, whether neat or in solution. It slowly distilled at 60-70" (0.01 Torr) to give a pale yellow oil which darkened on standing. The pH of a saturated aqueous solution was 0.1 unit above that of control water, and its solubility in water was 25 mg/l. at pH 5.5-7.0, 60 at pH 10.0, and 35 at pH 2.0 with slight darkening. It migrated on silica gel only in the presence of acid or base: ir 3450 (OH), 30032900 (CH,,), 1543, 1359 (NOz) c m - l ; nmr 6 8.5 ( s , Ph). 8.2 (s, P h ) , 4.3 (t. CHz), 3.3 (quadruplet. CHz), 1.7 (m, CHZ), 1.5 (t, CH3), 1.0 (t, CH3); mass spectrum m / e 317 (maximum), 301 (max - lo), 258, 213 (base). Anal. Calcd for C13H16N304F3: C, 46.6; H , 4.81; X, 12.5. Found: C, 47.9; H, 4.80; N,12.6. Thermal and Photochemical Decomposition of 11. I1 decomposed above 70". Three major volatile products were formed by heating it above 200" in a glass vessel or hot glc column and were separated by tlc (solvent A) and purified by glc (1% SE-30). They were collected as viscous liquids (Table 111) with the following spectra. Compound 111. See the section on standards. Compound IV. Ir 3370 (OH), 3135-2907 (CH), 1546, 1337 (NOz) cm-I; nmr 6 8.3 (s. Ph). 8.2 (s. Ph). 5.2 (quadruplet, OCH), 4.4 (t, CHz), 3.0 (m, CHz), 1.8 (d, CH3), 1.6 (t, CH3); mass spectrum m / e 315 (parent), 260 (base); exact mass calcd for C13H14F3N303, 317.0987; found, 317.0933. Compound V. Ir 3115-2907 (CH,), 1709 (CO), and
PHOTODECOMPOSITION OF TRIF1,LRALIN
COCH,
CH=CHp OH
C2H5
Figure 2.
Ill
IV
Figure 1 .
v
VIII
IX
-
Thermal and photochemical degradation of V I I
Compound XI. See the section on standards. Irradiation of VI1 was the same as for 11. x
XI
Photodecomposition of trifluralin
1541, 1339 (NOz) c m - l ; nmr 6 8.1 (s. P h ) , 7.9 (s, P h ) , 4.5 (t, CH3), 2.7 (s, isolated CH3), 1.6 (m, CHz), 0.9 (t, CH3); mass spectrum m / e 315 (parent), 257 (base). An aqueous suspension of I1 was exposed to sunlight in the same manner as I. The extracted neutral products were subjected to tlc in solvents A and D, and the acidic products in solvents A, B, and E . Acidic Extracts After fractionation by tlc with solvent A, the single main band ( R f 0.05) was developed with solvent G (RI 0.34) and then solvent B (Rc 0.15). The purified material was crystallized from acetone-benzene and dried in L ~ C U Oa t 25" over phosphorus pentoxide and paraffin shavings to provide VI1 as white crystals, mp 215216" dec, sublimation 130" (0.01 Torr), which were stable at 25" in a dark desiccator or vial up to several months: ir 3436 ( N H ) , 2632, 3003-2849 (CHn), 1538, 1337 (XO2) c m - l ; mass spectrum m / e 275 (parent), 274 (P - H ) , 259 (P - 0), 255 (base) (P - OH). Anal. Calcd for C10HsS303F3: C, 43.6; H, 2.93; N,15.3. Found: C, 43.2; H , 2.9; bi, 14.7. Satisfactory nmr spectra were not obtained. VI1 reacted vigorously with diazomethane in ether to give primarily ,1 white solid (XV), mp 115-116": ir 30862890 (CHn), 1534, 1335 (NO*) c m - l ; mass spectrum mle 289 (parent), 270 (base); nmr 6 8.4 (s, Ph). 8.0 (s, P h ) , 4.3 (s, isolated CH:,), 1.1 (quadruplet, CHz), 1.5 ( t , CH3). XV partially decomposed to XI on hot glc columns. Thermal and Photochemical Decomposition of VII. VII formed four volatile products when heated above its melting point in a glass vessel or a hot glc column. They were separated by tlc (solvent D) and rechromatographed with the same solvent, and VI11 and XI were further purified by glc (2% SE-30). All were white solids (Table 111) with the following spectra. Compound VIII. Ir 3300 ( N H ) , 3100-2900 (CH,), 1701 (CO), 1538, 1320 (NOz) c m - l ; nmr 6 8.7 (s, Ph), 8.6 (s, Ph), 3.9 (s, N H ) , 2.8 (s. isolated CH3); mass spectrum m l e 273 (parent and base). Compound IX (minor). Ir 3195 broad (OH), 3100-2882 (CH,,), 1527, 1328 (NOz) c m - l ; nmr 6 8.4 (s, P h ) , 5.3 (quadruplet, OCH), 3.1 ( s , NH or OH), and 1.7 (d, CH3); mass spectrum m / e 275 (parent), 186 (base). Compound X . Ir 3367 ( N H ) , 3096-2890 (CH,), 1517, 1333 ( S O z ) cm I; nmr 6 8.4 (s, Phj, 8.0 (s, P h ) . 7.1 (d of d. CH). 6.7 (d of d , CHz), 5.9 (d of d, CHz), 2.9 (s, NH); mass spectrum m / e 257 (parent), 211 (base).
RESULTS
Trifluralin was very unstable to sunlight, especially in the presence of an organic solvent. Its homogeneous solutions in 10% aqueous methanol contained the monodealkylated photoproduct VI after only about 30 sec in light, accompanied by a color change from pale yellow to dark yellow-brown during several hours in which tlc revealed the formation of over 20 products. Suspensions of I in water gave the same products but a t a slower rate; VI became detectable within about l hr. The dinitrotoluidines VI and XI1 tentatively identified by Day (1963) were confirmed (Table II), but the others suggested by Harrison and Anderson (1970) specifically were not detected under our conditions. Within a few minutes, the principal photodecomposition products formed in aqueous methanol or water consistently were 2amino-6-nitro-cu,cu,n-trifluoro-p-toluidine (XIII) (plus the artifact XIV derived from its reaction with acetone) and two unusual, highly polar compounds characterized as 2ethyl-7-nitro-5-trifluoromethylbenzimidazole3-oxide (VII) 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-trifluoroand methylbenzimidazoline (11) (Figure 1 ) . Two other products appeared to be azoxybenzenes and will be described a t a later date. Identification of VII. Initial evidence for the structure of VI1 came from examination of its thermal breakdown products VIII, IX, X, and XI. XI was shown to be 2-ethyl7-nitro-5-trifluoromethylbenzimidazoleby analysis and comparison of its physical properties with those of' a synthesized standard (Figure 2). The carbonyl absorption of VI11 at 1701 c m - I suggested a methyl aryl ketone, as did the nmr singlet a t d 2.8. Considering the parent mass, the formation of XI from VII, and the N H absorption at 6 3.9, VI11 must be 2-acetyl-7nitro-5-trifluoromethylbenzimidazole.The olefinic protons in the nmr spectrum of the similar compound X com= 18 Hz, J\\ = 10 Hz, prised an AMX system where Jhqs and Jhl.4 = 2 Hz; considering the parent mass, S must be 2-vinyl-7-nitro-5-trifluoromethylbenzimidazole. The quadruplet a t 6 5.3 in the nmr spectrum of IX, equivalent to one proton, was considerably shifted from the normal position (6 3.0) for methylene protons of closely related standards such as XI and suggested the presence of an oxygen atom on the same carbon. These and the other spectral data indicated that IX must be 2-(cu-hydroxyethyl)-initro-5-trifluoromethylbenzimidazole. The thermal degradation products suggested that VI1 2-ethyl-7-nitro-5-trifluoromethylbenzimidazole3was oxide, and the spectra, molecular weight, and chemical properties all agreed with the N-oxide structure. For exJ. Agr. Food
Chern.. Vol. 22. No. 5 , 1974
845
LEITIS. CROSBY
OaN& u
Table IV. Uv Absorption Characteristics of Trifluralin Photolysis Productsa
N
Compd
A
nm
A,,,
I1 I11 IV V VI1 VI11 IX X XI
312 310 321 322 320 312 322 311
E
X
lo-‘
5.0 6.0 16.0 5.0 1.4 6.4 4.4 6.6
Amax,
nm
247 226 228 254 255 254 229 229 224
E
X
0.9 0.5 0.9 0.3
0.8 0.3 0.7
0.8 0.4
* Methanol solution.
Figure 3.
Thermal and photochemical degradation of
I1,
(DNTj amino acids (Crosby and Bowers, 1968), and VI1 proved to be identical with the compound prepared by photolysis of N-(2’,6’-dinitro-4’-trifluoromethylphenyl)-2aminobutyric acid (XVIj by the method of Neadle and Pollitt (1967),thus confirming its structure.
ample, tautomerism caused 2-carbomethoxy-5-nitrobenzimidazole 3-oxide to give a yellow solution in base and a colorless solution in acid (Luetzow et al., 19661; VI1 behaved identically, while the other benzimidazoles did not give color and did not dissolve in base. Methylation of benzimidazole 3-oxide was reported to give a mixture of 1-methylbenzimidazole 3-oxide and 3-methoxybenzimidazole (Hayashi et al., 1960), and reaction of VI1 with diazomethane likewise provided a major product XV whose molecular weight (289) coincided with monomethylation. Breakdown of XV to give XI on a hot glc column suggested that diazomethane had reacted with the hydroxylamine tautomer to give 2-ethyl-l-methoxy-4-nitro-6-trifluoromethylbenzimidazole (eq 1).
CF
CF
IIa
IIb
CF; I
HOOC-CH-C~H-
IIC VI1
xv The photodecomposition products of VI1 were the same as those produced by heat. Loss of oxygen from aromatic amine N-oxides is generally observed upon photolysis (Spence et al., 1970); for example, 1-benzyl-2-ethylbenzimidazole 3-oxide was deoxygenated by light (Ogata et al., 1970) to form the corresponding benzimidazole analogous to the observed formation of XI from VII. Light also induces rearrangement of aromatic amine N-oxides to alcohols and ketones, and photolysis of 2,3-cyclopentenoquinoline N-oxide gave a high yield of 1,2,3,4-tetrahydrocarbazol-4-one (Spence e t al., 1970) while pyridine-2-methano1 resulted from the uv irradiation of 2-picoline N-oxide (Hata and Tanaka, 1962). Accordingly, VI1 provided the alcohol IX (which dehydrated thermally to the olefin X) as well as the ketone VIII. The photolysis of N-2,4-dinitrophenyl derivatives of amino acids at 360 nm in aqueous media (Neadle and Pollitt, 1969) provides a convenient route to 5-nitrobenzimidazole 3-oxides with a variety of 2 substitutents. The procedure was found to be equally effective for synthesis of benzimidazole oxides from dinitrotrifluoromethylphenyl 846
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Food Chem., Vol. 22, No. 5, 1974
XVI
Identification of 11. The structure of I1 was suggested by that of VI1 as well as by the thermal breakdown products 111, IV, and V (Figure 3). Spectra and comparison with a synthesized standard proved I11 to be 2-ethyl-7nitro-1-propyl-5-trifluoromethylbenzimidazole.The spectra and molecular weight of IV excluded all but two struc2-ethyl-7-nitro-l-propyl-5-trifluoromethylbenzimtures: idazole 3-oxide and 2-(ct-hydroxyethyl)-7-nitro-l-propyl5-trifluoromethylbenzimidazole. Absence of a strong P 16 peak in the mass spectrum was inconsistent with the N-oxide structure (Tatematsu et a / , 1967) and, as with IX, the quadruplet a t 6 5.2 equivalent to a single proton ( a downfield shift of 2 6 ) indicated attachment of oxygen a t the methine carbon of the carbinol structure. Spectral data likewise indicated that V was the corresponding methyl ketone. The similarity of‘ the thermal decomposition products and mass spectrum of I1 to those of VI1 suggested that it, too, was a benzimidazole N-oxide, but its uv spectrum (Table IV) showed none of the 310-322-nm absorption characteristic of the related benzimidazoles and their oxides. Instead, the data were consistent with a saturated structure which readily underwent elimination of water (as in the mass spectrometer) to generate a benzimidazole N-oxide, and combustion analysis conformed to any of three plausible compounds (IIa-IIc). IIa, a form of nitronic acid, should be at least as acidic as a carboxylic acid (Nielsen, 1969); the slightly basic solution and weakly amphoteric properties of I1 were more consistent with the other structures. The absence of a singlet above 6 3.5 in the nmr spectrum precluded structure IIb; integration in-
PHOTODECOMPOSITION OF TRIFLURA1,IN
dicated the expected 16 protons, with the two hydroxyls masked by CH2 and CHJ absorption (Newsom and Woods, 1973), an interpretation which might be verified by deuterium exchange. Cyclic diol absorption (3450 c m - l ) in the ir spectrum of neat, anhydrous I1 supported the chemical and spectral evidence for IIc. Other Photodecomposition Reactions. Although the major products formed by irradiation of I were the same whether water or methanol was employed as solvent, the relative amount of I1 decreased and that of VI1 increased in water. The number and amount of minor, colored, polar products increased in the presence of methanol. Likewise, the ratio of products changed with pH. A colorless, strongly quenching band just above band 3 in the neutral extract of I irradiated a t pH 11.0 was collected after rechromatography twice with solvent D and glc (2% SE-30) as a white solid, mp 149-151”, identified by mixture melting point and its ir spectrum as 2-ethyl-7-nitro5-trifluoromethylbenzimidazole (XI). Within 24 hr, it constituted 70-80% of the total photoproducts, with 11, VII, and XI11 each accounting for an additional 5%. T o verify the reaction sequence (Figure l ) , several of the photolysis products were exposed to sunlight. The major photoproduct from VI, isolated from the acidic extract of an irradiated aqueous suspension by the procedure for the acidic extract from irradiated I, was a white solid identical with VI1 by tlc with solvents A, B, and E, thermal decomposition pattern on glc, and ir spectra. XI11 was present in the neutral extract. A suspension of I1 in water was completely degraded by sunlight in 4 days; tlc revealed the thermal degradation products 111, IV, and V based on RI values in solvents A and D. The acidic extract contained the benzimidazole N-oxide VI1 (10% yield), identified by tlc with solvents A, B, and E, and glc degradation pattern (2% SE-30). In turn, I11 and IV were themselves completely photolyzed by sunlight in 4 days. A suspension of VII in water remained 30-40% unreacted after sunlight irradiation for 4 days and was recovered from the acidic extract. The neutral extract gave one major product, whose identity remains unknown, and three minor products corresponding to the thermal degradation products VIII, IX, and XI based on R1 values in solvents A and D. Irradiation of XI showed it to be very stable to light; approximately 70% was recovered unchanged after 4 days. although two major and four minor products were observed by tlc in solvent D. VI1 was relatively stable at pH 11. and no XI was observed as a photolysis product. DISCL‘SSIOS The photodecomposition of trifluralin involves oxidative dealkylation. nitro reduction, and cyclization. The photochemical N-dealkylation. like that of monuron (Crosby and Tang, 19691, zectran, and matacil (Abdel-Wahab and Casida, 1967), appears t o be a free-radical oxidation by atmospheric oxygen (Sharkey and Mochel, 1959); the reduction in methanol represents the well-known abstraction of hydrogen atoms from the medium (Plimmer. 1970; Crosby and Hamadmad, 1971), although the mechanism of photoreductions in water is not clear (Crosby, 1972a). A modification of the free-radical mechanism proposed by Doepp (1971) to explain the photochemical formation of indole AV-oxidesfrom nitroaralkanes provides a plausible route from Znitroanilines to the benzimidazoles (Figure 4).The apparently facile abstraction of H atoms from the N-alkyl group which leads to dealkylation also should encourage the rapid formation of B (and perhaps C) from the light-excited A; McMahon (1966) reported that dinitroanilines such as VI are converted photochemically to aldehydes and 2-nitrosoanilines (E), logically through D, and Russell (1965) showed that the nitrosoanilines under-
R h
C -
G -
Figure 4. Proposed mechanism of dinitroaniline herbicide pho-
tolysis.
go aldol condensation with aldehydes under mild conditions to provide the corresponding benzimidazole N-oxides (F). The dehydration of F is exactly analogous to the synthesis of nitrones from ketones and N-substituted hydroxylamines (Exner, 1951). Where R = H, F should be easily dehydrated to the resonance-stabilized G, while it should be relatively stable when R = alkyl. The mechanism is quite general for the dinitroaniline herbicides. Those similar to I in their alkylation pattern should form type F compounds and benzimidazoles readily, as indeed does dinitramine (2,B-dinitro-N,N-diethyl-3amino-cu,a,cu-trifluoro-p-toluidine) (Newsom and Woods. 1973); where there is chain branching adjacent to the N, compounds such as C and E should predominate. as shown with dibutalin (N-sec-butyl-4-tert-butyl-2,6-dinitroaniline) (Plimmer and Klingebiel, 1972). The mechanism may be ionic as well as radical (Meth-Cohn and Suschitzky, 1972), especially after the first step, perhaps explaining the formation of XI in alkaline media. Considering the opportunities for dealkylation, reduction, and cyclization also available to the reactive intermediates of trifluralin photolysis, the eventual photodecomposition of the herbicide to a largely unresolvable mixture of numerous trace products is understandable. In the laboratory, the relative stability of three of the compounds-VII, XI, and XIII-became apparent, but the known reactivity of 1,2-phenylenediamines with sugars (Heath and Roseman, 1963), acids (Pool et al., 1937), and ketones (as demonstrated by formation of XIV during extraction and chromatography) suggests that XI11 might react readily with environmental reagents under field conditions. Although the photoreduction required for its formation predictably is enhanced by an organic medium, the reaction obviously proceeds even in pure water (Nakagawa and Crosby, 1971; Crosby e t al., 1972). The dealkylated benzimidazole XI, most stable of the photoproducts, might persist in the environment long enough to be detectable as was the case with the corresponding compound from dinitramine (Smith, 1972). At alkaline pH, XI became the principal trifluralin photolysis product, and exposure of I vapor to uv light provided XI. together with VI (Moilanen and Crosby, 1972). As in previous investigations with dinitroaniline herbicides, the present work was severely complicated by the presence of “polar products” such as I1 and VII. Polar compounds also were reported to represent the terminal decomposition products of I in both aerobic and wet anaerobic soils (Probst et al., 1967), together with VI, XII, XIII. and related amines. Considering the presence of free radicals in soil (Steelink and Tollin, 1967) to initiate the simple reactions of Figure 4 instead of light, it appears reasonable that benzimidazolines and benzimidazole A‘oxides may occur in trifluralin-treated soil, and the repeated similarity between the products of photodecomposition and of metabolism (Crosby, 1972b) suggests that the same may be true of the “polar products” found in J . A g r . Food Chem.. Vol. 22. No. 5. 1974
847
LElTlS. CROSBY
plants and rumen fluid (Probst and Tepe, 1969) exposed to the herbicide. The consistent thermal decomposition pattern of I1 and VII on glc could be utilized to detect their presence in the immobile fractions from tlc. Otherwise, however, the present investigation demonstrates clearly that reliance upon glc for identification, as in previous trifluralin investigations, can introduce artifacts and obscure the major photolysis products. ACKNOWLEDGMENT We are grateful to R. B. Foster, R. E. Lundin, D. W.
Thomas, and C. A. Reese for assistance with measurements, C. J . Soderquist for other technical assistance, D. L. Hunter, Alson Lee, and J . N. Seiber for helpful discussions, and H. C. Newsom for a prepublication copy of his manuscript. LITERATURE CITED Abdel-Wahab, A. M., Casida, J. E., J . Agr. Food Chem. 15, 479 (1967). California Department of Agriculture, “Pesticide Use Report,” Sacramento, Calif., 1970, 1972. Crosby, D. G., Aduan. Chem. Ser. No. 111, 173 (1972a). Crosby, D. G., “Degradation of Synthetic Organic Molecules in the Biosphere,” NAS-NRC, Washington, D. C., 1972b, p 260. Crosby, D. G., Bowers, J . B., J. Agr. Food Chem. 16,839 (1968). Crosby, ,_ D. G., Hamadmad, N., J . Agr. Food Chern. 19, 1171 ( 1 Y 7 11.
Crosby, D. G., Moilanen, K . W., Kakagawa, M., WOW, A. “Environmental Toxicology of Pesticides,” Matsumura, F., Boush, G. M., Misato, T., Ed., Academic Press, New York, w. Y., 1972, p 423. Crosby, D. G., Tang, C. S..J. Agr. Food Chem. 17,1041 (1969). Day, E . W., Jr., Eli Lilly and CO., Indianapolis, UnPublished work, 1963. Doepp, D., Chem. Ber. 104, 1043 (1971). Emmerson, J. L., Anderson, R. C., Toricol. A p p l . Pharmacol. 9, 84 (1966). Exner, O., Collect. Czech. Chem. Commun. 16,258 (1951). Hall, R. C., Choo-Seng, G., J. Agr. Food Chem. 20,546 (1972). Harrison, R. M., Anderson, 0. E., Agron. J . (i2, 778 (1970). Hata, N., Tanaka, I., J . Chem. Phys. 36,2072 (1962). Hayashi, E., Ishiguro, E., Enamoto, M., Abstracts, 80th Meeting of the Pharmaceutical Society of Japan, 1960. Heath, E . C.. Roseman. S., Methods Carb0h.d. Chem. 2, 138 (1963). Leitis, E., Crosby, D. G., Abstracts, Western Regional Meeting of the American Chemical Society, San Francisco, Calif.. Oct 1972.
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Luetzow, A . E., Hoffman, A. E., Vercellotti, J . R.. Chem. Cammun., 301 (1966). McMahon, R. E., Tetrahedron L e t t . , 2307 (1966). Messersmith, C. G., Burnside, 0. C., Lavy, T. L., Weed Sci. 19, 285 (1971). Meth-Cohn, O . , Suschitzky, H., Aduan. Heterocycl. Chem. 14, 21 1 (1972). Moilanen, K . W., Crosby, D. G., Abstracts, 163rd National Meeting of the American Chemical Society, Boston, Mass., April, 1972. Nakagawa, M., Crosby, D. G., Abstracts, 162nd National Meeting of the American Chemical Society, Washington, D. C., Sept 1971. Neadle, D. J., Pollitt, R. J . , J. Chem. SOC.C, 1764 (1967). Neadle, D. J., Pollitt, R. J., J . Chern. SOC.C, 2127 (1969). Newsom, H. C., Woods, W. G., J . Agr. Food Chem. 21, 598 (1973). Nielsen, A. T., “The Chemistry of the Nitro and Nitroso Groups, Feuer, H., Ed., Interscience, New York, N. Y., 1969, Chapter 7. Ogata, M., Matsumoto, H., Takahashi, S., Kano, H., Chem. F‘harm. Bull. 18,964 (1970). Patai, S., Zabicky, J., Handbook of Chemistry and Physics,” Weast, R. C., Ed., 51st ed, Chemical Rubber Co., Cleveland, Ohio, 1971, p C216. Plimmer, J. R., Residue Reu. 33, 47 (1970). Plimmer, J . R., Klingebiel, U. I., Abstracts, 164th National Meeting of the American Chemical Society, Kew York, N. Y., Ort - - - 1972 - - . -. Pool, W. O., Harwood, H . J., Ralston, A . W., J . Amer. Chem. SOC.59,178(1937). Probst, G. W., Golab, T.. Herberg, R. J., Holzer, F. J., Parka, S. J., Van der Schans, D., Tepe, J . B., J. Agr. Food Chem. 15, 692 (1967). Probst, G. W., Tepe, J. B., “Degradation’of Herbicides,” Kearp. C., 255, Kaufman, D. D., Ed., MarcelDekker, NewYork, h’. ney. y . , 1969, Russell, D , w , , chern,c ~ 498 ~ (1965). ~ ~ ~ , , Sharkey, W. H., Mochel, W. E., J . Amer. Chem. SOC.81, 3000 (1959). Smith, R. A,, Abstracts, 164th National Meeting of the American Chemical Society, New York, N. Y., Aug 1972. Spence, G. G., Taylor, E. C., Buchardt, O., Chem. Reu. 56, 231 (1970). Steelink, D., Tollin, G., “Soil Biochemistry,” McLaren, A. D., Peterson, G. H., Ed., Marcel Dekker, New York, N. Y., 1967, p 147. Tatematsu, A,, Yoshizumi, H., Hayashi, E., Nakata, H., Tetrahedron Lett., 2985 (1967). Wright, W. L., Warren, G. F., Weeds 13,329 (1965). Received for review August 20, 1973. Accepted May 3, 1974. Presented at the Western Regional Meeting of the American Chemical Society, San Francisco, Calif., Oct 1972. Supported in part by National Institutes of Health Grant ES-00054 and U. S. Department of Agriculture Regional Research Project W-45.
